Electrically conductive polymers have been found to be useful in electronic devices such as light-emitting diodes (LEDs), photodetectors and photovoltaic cells. It is well known to use a layer of conductive polymer, such as poly(3,4-ethylenedioxythiophene) (PEDOT), between the inorganic anode and the light-emitting layer. The conductive polymer layer is variously referred to as part of a bilayer anode, a hole-injection layer or a buffer layer. Such systems have been described in, for example, Jonas et al., U.S. Pat. No. 5,766,515.
Useful synthetic procedures for the preparation of a poly(dioxythiophene) such as PEDOT are well known. For example, the dioxythiophene monomer can be treated with ammonium persulfate or potassium peroxydisulfate in excess hydrochloric acid in water. Such reactions are known as oxidative polymerization, wherein monomers such as thiophenes are oxidized in the presence of a protonic acid. This reaction results in a positively charged poly(dioxythiophene), with the charge being balanced with the anion from the acid. Such processes have been described in U.S. Pat. No. 5,035,926, U.S. Pat. No. 5,300,575, U.S. Pat. No. 6,083,635 and European Patent application 440 957.
The thickness of the conductive polymer layer needed in the above-mentioned electronic devices depends to some extent on the surface roughness of the conductive layer. Thicker layers are needed as the surface roughness increases. In order to prepare smooth and uniform layers, it is desirable to have dispersions of the conductive polymers with smaller particle size. Moreover, it is desirable to find new polymer systems with electrical properties that provide better performance in the conductive polymer layer.